The invention relates to a process for decontamination (phytoremediation) of materials such as soils, sludges, sediments, wastes, industrial deposits and for continuously keeping clean, on a preventative basis, filter bodies consisting of these or similar material (phytoprevention) for clean-up of waste water such as roof runoff, road runoff, community sewage, industrial waste water which is loaded with at least one pollutant from the group of heavy metals, metalloids, halides and/or radionuclides, in which the pollutants are removed from the material by phytoextraction by means of woody plants which extract the pollutants to be removed from the contaminated material from the soil and accumulate it in the leaves. In particular, the process is intended for in situ decontamination of pollutant-loaded soils, sludges, sediments or similar materials.
The absorption of pollutants in plants using preferably deep-rooted species represents an environmentally non-destructive technology for removal of pollutants, especially heavy metals and radioactive materials, from soils and similar materials.
U.S. Pat. No. 5,927,005 A relates to a process for decontamination of soils which contain heavy metals. Here phytoremediation (phytorehabilitation) is to be used. In particular metals such as copper, nickel, and cadmium will be removed from soils using perennial plants.
In doing so the metals to be removed from the soil will accumulate in unimportant tissue parts of the plants so that the plants can continue to be used.
In the phytorehabilitation described in U.S. Pat. No. 5,927,005 A bushes will be either cut completely down or partially cut back to collect the metals which have accumulated in the bushes. This is done especially once a year. In order to obtain metals which have accumulated in the leaves and stems, the dried plant material can also be extracted with an aqueous acid according to the process of U.S. Pat. No. 5,927,005 A.
U.S. Pat. No. 5,320,663 A describes a process for obtaining lead and organo-lead compounds from contaminated materials using metal-accumulating plants. In the process of U.S. Pat. No. 5,320,663 A the plants will be harvested in whole or in part in order to obtain the lead which has accumulated in them. Here the volume of the harvested plants or plant portions will be reduced if necessary after reduction of volume by incineration, anaerobic or aerobic decomposition, acid digestion or composting and then the plants or portions of them will be subjected to a heat treatment process (smelting).
Usually the contaminated plant material is seasonally harvested in the known processes and the resulting harvest residues are composted or burned; both are associated with relatively high costs (compare U.S. Pat. No. 5,320,663 A). The xe2x80x9chandlingxe2x80x9d of contaminated materials leads to additional exposure of workers (and the environment) to pollutants. Another problem is that the periodic or seasonal harvesting of plant biomass (compare U.S. Pat. No. 5,320,663 A) and U.S. Pat. No. 5,927,005 A) leads to plants"" dying off or under certain circumstances to a two to three year regeneration period and the process of phytoextraction is interrupted for this time interval.
Processes of phytoextraction known to date are often based on annual plants with either high pollutant accumulation and low biomass (hyperaccumulators, for example Thlapsi caerulescens) or with relatively low pollutant accumulation and high biomass (for example, Brassica juncea). To increase pollutant removal in the latter case, chelate-forming agents such as ethylene diaminotetraacetic acid (EDTA) or nitritotriacetic acid (NTA) are used, which in addition result in high costs and the possibility of long-lasting leaching of pollutants into the groundwater. Especially for heavy metals and radionuclides thus no nondestructive, economical and ecologically acceptable rehabilitation technologies are available with which pollutants can be removed from contaminated material.
The object of the invention is to make available a process of the initially mentioned type which is simple to carry out and is still effective.
This object is achieved with the inventive process which comprises the following process steps:
b) application of a layer of material which chemically and/or physically binds at least one pollutant on the soil surface in an area of woody plants;
c) collection and composting of the natural leaf fall on the layer;
d) leaching of the pollutant out of the leaves decomposing on the layer; this leaching can also and especially be passive leaching, for example by precipitation, such as rain, dew, etc.;
e) physical and/or chemical binding of the pollutants in the layer in order to interrupt the bio-geochemical circulation of the pollutants and in order to enable regeneration of the material of the layer; and
f) removal of the layer after the goal of decontamination has been reached.
Following step f) the material of the layer can be eluted for example with a solvent (such as ethylene diaminotetraacetic acid=EDTA) for recovery of the pollutants and for further concentration of the pollutants and for regeneration of the material of the layer.
The process of the invention is carried out using perennial, generally woody plants with high biomass production and the ability to accumulate pollutants in the leaves in high concentrations. In particular, suitable plants are representatives from the family of willows (Salicaeceae), poplars (Salicaeceae) and birches (Betulacaea).
The process of the invention is based on a novel combination of phytoextraction (absorption of pollutants by the plants and transport into the above-ground parts of the plant such as the leaves, branches and trunk), pollutant transport with falling of the leaves to the soil surface, leaching of the pollutants out of the leaves decomposing on the soil surface and (physico-)chemical fixing of the leached pollutants in a reactive layer usually produced before planting, bounded for example by geotextiles, with a composition (for example clay minerals, oxides) and layer thickness which can be matched to the type and degree of burden of the pollutants to be fixed and the expected remediation interval.
For example, embodiments of the process of the invention are explained below.
Phytorehabilitation (phytoremediation) is a promising rehabilitation method which uses the ability of selected plant species or their ecotypes/varieties to fix pollutants in contaminated soils either in a form insoluble in water or to remove them from the soils (or waste waters).
The process step of the invention in which the pollutants are removed from the contaminated (soil) material (or sediments, sludges, wastes, industrial dumps, etc.) by transport into the plant biomass and accumulation in it is called phytoextraction.
The disadvantages of the known processes of phytoremediation of contaminated materials (soils, sludges, sediments, wastes, etc.) are eliminated with the inventive process by the phytoextraction being combined with (physico-) chemical fixing (immobilization) of the pollutants after natural leaf fall in the inventive process. Thus the interruption of phytoextraction caused by harvesting is avoided and the efficiency of the phytoextraction can be increased. To bind the pollutants at least one layer which physically and/or chemically. binds the pollutants is applied to the soil surface to fix the pollutants which have leached out of the decomposing leaves, branches and limbs (xe2x80x9cpollutant trapxe2x80x9d).
The layer which physically and/or chemically binds the pollutants can be made such that (physico-)chemically reactive materials with a layer thickness of preferably a few centimeters are bounded like a sandwich by geotextiles with a long lifetime. The layer which physically and/or chemically binds the pollutants can, depending on the extent and type of contamination, be structured such that it remains usable for twenty years or longer for pollutant fixing.
The layer which physically and/or chemically binds the pollutants after preparation of the terrain can be delivered by hand or mechanically (in the case of prefabricated products, for example in a sandwich-like structure geotextilexe2x80x94reactive materialxe2x80x94geotextile) or using spreading or spraying devices (for example, a clay mineralxe2x80x94bitumenxe2x80x94strawxe2x80x94mixture).
The layer which physically and/or chemically binds the pollutants after the rehabilitation interval or when its pollutant fixing capacity is exhausted can be removed for example by tractors with scraping and lifting devices. The layer which physically and/or chemically binds the pollutants and which has accumulated the pollutants can be regenerated on site or after transport to secured facilities (for example, a dump) with recovery and further concentration of the pollutants (for example by washing (elution) with solvents, for example EDTA, and recovery of heavy metals), or according to the pertinent legal regulations at a volume which has been greatly reduced compared to the originally contaminated material, it can be safely and relatively economically dumped.
During the entire remediation interval (in many cases several decades) some of the costs of the process can be balanced by the case-by-case and/or partial harvesting and use of woody biomass to produce renewable raw materials (biofibers, paper/cardboard, industrial alcohol, biospirit, etc.) or energy. The inventive process thus enables continuous use of the plants during remediation.
The inventive process can be carried out on site (in situ), but is not limited to this version.
Pollutants which can be removed with the inventive process from contaminated material comprise heavy metals (for example, Pb, Cd, Zn), metalloids (for example, As) and radionuclides (for example, Sr).
Preferably suitable plants are mainly fast growing, woody representatives of the plant families of willow growths (Salicaeceae), poplar growths (Salicaeceae) and birch growths (Betulaceae). Varieties, clones and ecotypes with the ability to accumulate at least one or more pollutants, especially heavy metals such as Pb, Cd and Zn in the leaves, especially in high concentrations, are used.
Suitable materials for the layer which physically and/or chemically binds the pollutants comprise materials or material combinations with high binding capacity for the pollutant(s) under consideration, especially clay minerals (for example, vermiculite), oxides of iron, aluminum, or manganese and poorly soluble phosphates (apatites). It is advantageous that in the inventive process also residual materials from industrial processes can be used.
The process of phytoextraction can be used not only for decontamination of solid (phytoextraction in a narrower sense) or liquid (rhizofiltration) media after contamination with pollutants exceeding legal and/or scientifically sound rehabilitation thresholds for phytoremediation (phytorehabilitation). The inventive process can also be carried out in the sense of phytoprevention. In this embodiment the process is used for continuous preventative cleanup of soil filters (or filter bodies and similar materials) which are used to purify waste water. In doing so the amount of pollutants delivered via the waste water to the filter within one year (summer and winter) is removed again by the plants during the vegetation period and accumulated in the leaves. The soil filter can thus remain permanently in use since its capacity is not exhausted. The contaminated leaves can be further treated, and decomposition (composting) can take place in situ or ex situ (but conventionally on-site).